U.S. Pat. No. 4,510,489, issued to Anderson et al., discloses a magnetomechanical electronic article surveillance (EAS) system in which markers incorporating a magnetostrictive active element are secured to articles to be protected from theft. The active elements are formed of a soft magnetic material, and the markers also include a control element which is biased or magnetized to a pre-determined degree so as to provide a bias field which causes the active element to be mechanically resonant at a pre-determined frequency. The markers are detected by means of an interrogation signal generating device which generates an alternating magnetic field at the pre-determined resonant frequency, and the signal resulting from the mechanical resonance is detected by receiving equipment.
According to one embodiment disclosed in the Anderson et al. patent, the interrogation signal is turned on and off, or "pulsed," and a "ring-down" signal generated by the active element after conclusion of each interrogation signal pulse is detected.
Typically, magnetomechanical markers are deactivated by degaussing the control element, so that the bias field is removed from the active element thereby causing a substantial shift in the resonant frequency of the active element.
FIG. 1 is a somewhat schematic, exploded isometric view of a magnetomechanical EAS marker of the type disclosed in the Anderson et al. Parent. In FIG. 1, reference numeral 20 generally indicates the magnetomechanical marker. The marker 20 includes a housing 22 which defines a recess 24 in which the magnetostrictive active element (reference numeral 26) is housed. A bias or control element 28 is secured to the housing 22 at a position adjacent to the active element 26. As seen from FIG. 1, both the active and bias elements are in the form of thin, planar, ribbon-shaped strips of materials having magnetic characteristics suitable for the respective functions of the two elements. Conventional materials used for the active and bias elements are metal alloys.
FIG. 2 illustrates typical resonant frequency and output signal amplitude characteristics exhibited by a known magnetomechanical EAS marker, as functions of the effective bias field applied to the active element 26 by the bias magnet 28. In FIG. 2, curve 30 shows a bias-field-dependent output signal amplitude characteristic. Curve 30 is to be interpreted in conjunction with the right-hand vertical scale in FIG. 2. Specifically, curve 30 represents the so-called "A1" signal, which is the output signal level measured one millisecond after termination of an interrogation signal pulse. It will be observed that a peak value for the A1 signal occurs at a bias field level that is between 6 and 9 Oe.
Curve 32 in FIG. 2 indicates how the resonant frequency of the active element 26 varies according to the level of the effective bias field provided by the bias magnet 28. For the purposes of FIG. 2, the bias field is measured in the longitudinal direction of the marker, which is also the longitudinal direction of both the active element 26 and the bias magnet 28. Curve 32 is to be interpreted with reference to the left-hand vertical scale in FIG. 2.
In known magnetomechanical EAS markers it is customary to provide a bias magnet such that the effective bias field along the length of the active element is fairly close to the peak A1 signal level. In a typical magnetomechanical marker, the bias field provided by the bias magnet is about 6 Oe when the marker is in an active condition. In addition, the bias field level should be such that substantially degaussing the bias magnet, thereby reducing the applied bias field to a level of 2 Oe or below, results in a substantial shift in the resonant frequency of the active element, as well as a substantial reduction in the A1 output signal level. The resonant frequency shift, together with reduction in output signal level, helps to assure that the marker is "deactivated" i.e. that the marker will not be detected by the detection device provided at a store exit.
FIG. 3 presents in another form data represented by the resonant frequency characteristic curve 32 of FIG. 2.
The various data points shown in FIG. 3 correspond to respective bias field levels. The vertical position of each data point in FIG. 3 corresponds to the total shift in marker resonant frequency (deactivation frequency shift, or "DFS") if the bias field is reduced to 2 Oe from the respective bias field level corresponding to the data point. The horizontal position of the data point corresponds to the slope of curve 32 at the respective bias field level. (As a practical matter, for a given bias field level, the slope may be measured by applying a 0.5 Oe field in a first lengthwise direction of the marker and then in the opposite lengthwise direction, and noting the resulting difference in resonant frequency.)
The data shown in FIG. 3 indicates that the deactivation frequency shift, which is a desirable characteristic and is represented by the vertical scale, is positively correlated with the resonant-frequency-curve slope, which is represented by the horizontal scale and is a quantity that is to be minimized. The total frequency shift should be maximized, in order to minimize the possibility that a supposedly "deactivated" marker would be detected by detection equipment. On the other hand, the resonant-frequency-curve slope should be minimized, in order to reduce the chance that an "active" marker would fail to be detected. As discussed in U.S. Pat. No. 5,568,125, issued to Liu (and commonly assigned with the present application), the resonant frequency curve slope should be minimized to reduce the sensitivity of the marker to variations in the bias field. Bias field variations may arise due to manufacturing variations in regard to the bias magnet or other marker components, or as a result of the net additive or subtractive effect of the earth's magnetic field, depending on the orientation of the marker. To the extent that a marker is sensitive to bias field variations, the resonant frequency of the marker may be shifted from the nominal operating frequency of the detection equipment and may therefore be less likely to be detected by the detection equipment.
The positive correlation of DFS and resonant-frequency-curve slope, as indicated by FIG. 3, indicates that a trade-off must be made between reliable marker deactivation, provided by maximum DFS, and reliable marker detection, resulting from minimal sensitivity to bias field variations.
The Liu '125 patent, and co-pending patent application Ser. No. 08/800,771 (which is also commonly assigned with the present application) teach certain techniques for annealing the magnetostrictive active element and/or selecting the material of which the active element is formed, to ameliorate the trade-off between the desirable characteristic of maximum DFS, and the undesirable characteristic of elevated resonant-frequency-curve slope. It would, however, be attractive to provide additional techniques for ameliorating this trade-off, and it would be particularly helpful to improve this trade-off in a case where the active element is of a material that is used "as-cast", i.e. without annealing.